1. Field of the Invention
The present invention relates to a composition of a skin patch preparation comprising a hydrophilic synthetic polymer gel, which can homogeneously gel at any desired crosslinking rate, has tack strength according to the purpose, does not change with the lapse of time, and a process for preparing the same.
2. Description of the Prior Art
The following physical properties have hitherto been required of a composition of a skin patch preparation.
(1) Tackiness: it can be well applied to the skin, can conform to the movement of the affected part, and enables this tackiness to persist for a long period of time.
(2) Insolubility: it can gel to a suitable extent and, even when perspiration occurs, becomes neither sticky nor softened.
(3) Peelability: it can be peeled without leaving any external preparation.
(4) Stability; it neither hardens nor becomes sticky upon a change in temperature or with the lapse of a long period of time.
(5) High water content: it can contain a large amount of water (the larger the water content, the better the effect of cooling the affected part).
A composition prepared merely by mixing aqueous solutions of several water-soluble polymer materials, such as gelatin, is among conventional compositions of a skin patch preparation. This composition cannot, of course, satisfy the above various characteristics. For example, an attempt has been made to react gelatin with glutaraldehyde or urea to insolubilize the composition. In this case, the crosslinking rate is excessively high, or otherwise the crosslinking occurs only under given conditions of temperature, pH and the like, making it difficult to prepare a composition of an external preparation having a satisfactory degree of crosslinking. Therefore, no composition of an external preparation which can satisfy the above various characteristics can be provided. A gel prepared by crosslinking a water-soluble polymer of an aliphatic carboxylic acid or a salt thereof with aluminum is known to have excellent characteristics. The incorporation, however, of a polyvalent metal ion in an amount exceeding a specified value under stirring into a viscous liquid containing a water-soluble polymer of an aliphatic carboxylic acid or a salt thereof causes an immediate reaction to thereby give rise to a heterogeneous flocculent precipitate, thus deteriorating the usefulness as a thickening agent.
The applicant has proposed, in Japanese Patent Publication No. 41926/1986, a process for preparing a gel usable in a composition of an external preparation by crosslinking a water-soluble polymer of an aliphatic carboxylic acid or a salt thereof with aluminum. Although this process can provide a homogeneous gel, a reaction at a desired rate is difficult, posing such problem that it is difficult to control a change in quality with the lapse of time. Japanese Patent Publication No. 33685/1991 discloses a method wherein crosslinking can be conducted at any desired crosslinking rate. In this method, however, the control system is complicated and an unnecessary metal ion is incorporated, so that the resultant gel is not always satisfactory in the prevention of a change in quality with the lapse of time and irritation to the skin, making it impossible to always prepare a composition of an external preparation having a constant quality.
Skin patch preparations, for example, fomentations for application to the skin, are required to have various characteristics, such as shape retention, water retention, and stability without causing a change in quality with the lapse of time. For this reason, in order to impart or maintain these various characteristics, a polymer gel has been used as a base for fomentations. In the preparation of such fomentations, in order to always provide products having a constant quality free from a change with the lapse of time, when such factors as the working time necessary for processing and the time necessary for coping with a sudden accident are taken into consideration, it is necessary that the viscoelasticity is large enough to permit working persist for 1 to 5 hr, that gelation occurs immediately after the preparation that the-reaction is completed in a short time, and that after being put on the market, the crosslinking continuously proceeds to prevent a change in quality with the lapse of time.